Electronic switching system in a diversity receiving system



Dec. 27, 1949 Filed May 8, 1946 J B. ATWOOD DIVERSITY RECEIVING SYSTEM ELECTRONIC SWITCHING SYSTEM IN A 2 Sheets-Sheet l INVENTOR ATTORNEY Dec. 27, 1949 woo 2,492,780

ELECTRONIC SWITCHING SYSTEM IN A DIVERSITY RECEIVING SYSTEM Filed May 8, 1946 2 Sheets-Sheet 2 INVENTOR ATIORNEY hereinbef ore.

Patented Dec. 27, 1949 v UNITED STATE I PATENT OFFICE ELECTRONIC SWITCHING SYSTEM IN A DIVERSITY RECEIVING SYSTEM John B. Atwood, River-head, N. Y., assignor to Radio Corporation of Delaware Application May 8, 1946, Serial No. 668,109

7 Claims. (Cl. 250--8) This application relates to telegraphyand the like signalling systems and more in particular to frequency shift telegraphy or facsimile receivers in space diversity. Y Telegraphy and facsimile systems makinguse. of the frequency shift type of modulation are known in the art. In these systems alternating current has its frequency shifted from a first value which might represent mark" in teleg- .raphy or white in facsimile to a second value which might represent space in telegraphy or black in facsimile. At the receivers, this current is subject to a demodulation process to derive other current; for example, currenthaving a strong direct current component "which shifts in magnitude between two values one of which represents mark or white and theother of which represents space or black. These currents then are used to'operate directly or indirectly (in many cases by way of tone keyers) America, a corporation of which is distinctive as compared to the magnitudes of the others thereof and indicates which of the three versions of the signal is best. This summation of the locking circuit outputs takes .place in what I have designated coincidence tube pairs which in turn control the gating tubes.

Since the common outputof the gating tubes is connected directly, or by way of auxiliary equipment which may include a tone keyer, to recording apparatus it is essential that only that gating tube be opened which will allow the best signal recording apparatus of some type; for example,

teletypewriters. Arrangements of this nature are shown in Schock et a1. U. S. application #632,978 filed December 5, 1945 and in Schock U.S. application Serial #630,429 filed November 23, 1945, now abandoned. In these disclosures, two re-"' ceivers are included in the diversity systems.

signals for recording purposes than a single receiver. still better results and an object of the present invention is to provide triple diversity in frequency shift systems of the type outlined briefly In the diversity systems referred to strength or signal quality and are subject to a sensing operation wherein the best of'the' two signals is sensed and used for recording purposes.

A further object of the present invention is to; provide an improved method of and means for comparing three versions of the transmitted signals and sensing the best thereof to use for recording purposes. In the receiver of U. S. application Serial- #632,978 gating tubes are used to make available for recording the best signal only and for blocking oil the remaining signals. In the said application, the gating stages are in a sense differential and are differentially controlled by a lock; 'ing circuit arrangement set into operation by a potential which represents the difference in magnitude of the two versions of the signals. An object of the present invention is an improved gating tube controlling circuit including three Three receivers in space diversity give to hereinbefore, the transmitted currents are compared as Two receivers in diversity usually provide better to pass. A gating tube bias clamping arrangement is provided wherein diodes are connected in shunt to a portion of the control grid to cathode impedance of each gating tube and so poled as to drain off or eliminate all potentials applied thereto which exceed a critical value.

Indescribing my invention in detail reference will be made to the attached drawings wherein the single figure illustrates by circuit element and circuit element connections a frequency shift system including receivers in triple diversity arrangedin accordance with my invention.

.In the drawing, I have shown three receivers labeled .set A, set B, and set C connected by leads 2,4, and 6 to, pairs of signal quality sensing amplifiers and detectors labeled generally SS, S'S and S"S.

The receiversets A, B, and C may each comprise radio frequency amplifiers 8, 8 and 8 and if desired a first heterodyne detector and local oscillator. Output of intermediate frequency is fed to intermediate frequency amplifiers Ill, 10' and l0 In these amplifiers a second heterodyning process may be carried out if desired so cycles to 50.4 kilocycles. here my assignees standard radio frequency amplifier and converter and intermediate frequency that at the output I derive frequency shifted oscillatory energy of low intermediate frequency. The oscillatory energy may beof say about 50 kilocycles mean frequency and may be shifted several hundred cycles, for example from 49.6 kilo- I may use as receivers amplifier e tc. or any other type receiver having the desired characteristics. The receivers include common automatic gain control circuits. The AqC circuits per'se are not shown or described quency shifts due to modulation thereon converted to corresponding variations in amplitude or magnitude and these variations are detected to derive at the discriminator detector outputs demodulation currents having a strong direct current component which changes in polarity, the polarity of the direct current component representing different signalling conditions. The output is filtered in units 20, 20" to remove the intermediate frequency and is then coupled through condensers D, D and D" so that on the leads '24, 24', and 24" I provide an alternating current signal potential which appears across resistors 26, 26 and 26" and on the injector grids 28, 28"and 28 of three tubes of the pentagrid type designated gate A, gate B, and gate C; the letters of the said designations correspond to the set letter applied to the three receivers. Moreover these potentials appearing across the resistors 26, 26' and 26" are of a polarity which varies in accordance with the signal modulations. These potentials might vary between a potential which produces about zero potential on the grid of the gate tube (with respect to its cathode) representing one signalling condition and say a potential which produces a high negative value on the grid of the gate tube (with respect to its cathode) representing another signalling condition so that the tubes gate A, gate B or gate C being supplied with this potential act as amplifiers when the signal swings from zero potential to the said negative potential. In this respect note that the cathodes of the gate tubes are held positive relative to ground by their connections to a positive point on potentiometer resistance PR but the grids are still negative with respect to the cathodes because they are connected by resistors 26, and 26" to a less positive point on the potentiometer PR. The biases on the grids 29, 29 and 29" are modified by gating tube control potentials supplied from the coincidence tube pairs I20, I20 and 120', described hereinafter,

in such a manner as to make that gate tube opergate A, gate B and gate C are tied in arallel and connected by load impedance 3.2 to the positive terminal of a direct current source the negative terminal of which is grounded. The high signal potential end of this load impedance 32 is connected to an amplifying coupling tube in unit 44 which feeds the signal modulations passed by the conductive gate tube through ,a low pass filter in unit 45 and through a double triggering circuit in unit 46 to a tone keyer in unit 41 from which the output may be supplied to recording means such as a teletypewriter. The limiter, discriminator and detector in each of the units of three receivers having the best signal.

-means' for sensing which signal is best and for 4 20, 20' and 20 per se are not claimed in this application as being novel and in order not to unnecessarily lengthen this disclosure, will not be described in detail herein. The tone keyer may be conventional and no description thereof will be given here.

As brought out hereinbefore one of the purposes of my invention is to take advantage of space diversity effects and for these reasons I have provided means for selecting signal from that one This supplying a control potential for opening the appropriate gate tube will now be described. signals at intermediate frequency from the amplifier It are supplied by lead 2 in parallel to the grids of two amplifiers 60 and s2. The cathodes of these amplifiers are grounded through self bias resistors bypassed for the intermediate frequency and the other side of the output of amplifier I0 is grounded so that intermediate frequency energy modulated in frequency by the signals is impressed. on the input electrodes of tubes 60 and 62. The anodes of these tubes are connected to the primary'windlngs of output transformers 64 and 66. The secondary winding of transformer 64 is connected to diode rectifier 68 while the secondary winding of transformer 66 is connected to a diode rectifier iii. The rectifier 68 circuit has as a load impedance a load resistor 14 between the anode and the secondary winding of transformer "rectifier 10' in a circuit including a load impedance 16. Tubes GE!" and 62" with transformers .54" and 66 are similarly connected with rectifiers 68" and 1B" in circuits including load impedances 14" and 16'. The load impedances are shunted by bypassing condensers 11, TI, 11", 18, i8, and 18" which filter out or bypass all potential variations of signal frequency and higher. Potential variations which represent the mean or average carrier strength and lower are not filtered out.

The problem is to compare the strengths of three signals and sense which signal is the strongest. To do .so each signal must be compared with the other signal. This is accomplished in my system by the units SS, S'S and S"S and the rectifiers associated therewith. A rectifier say 68 excited by the output of receiver set A is .connected in series with a rectifier say 79" excited by the output of receiver set C. This series circuit includes rectifier load impedance (4, lead 80,

grid resistor 86', voltage dropping resistor '81",

rectifier load impedance 16", lead 89 and :load impedance 14. The output impedance .16 of .rec-

tifier 7.0, excited by output from set A, is similarly connected inseries, by lead a, resistor 81, resistor 86 and lead 15b, with the output impedance ld'of rectifier 68, which is excited by output from the receiver set B. The output impedance 16' ,of

nected in series with the output impedance 14" of rectifier ,68 excited from the output of set 'C by lead 'l5'a, resistor 81', resistor 86' and lead b'ludingtubes I00, I02 and I00,

Iijb. "Theload' impedance and rectifier arrangements are such that the outputs of the rectifiers having theirjloads inseries are opposed so that in each of the seriesrectifier output circuits potentials representing the magnitudes of the signals from two receiver sets are opposed to leave a diifere'ntial'resultant potential the polarity of which depends on'which of the two compared signals is the stronger. r

These potentials are across the grid to cathode input impedances of. trigger driver tubes 92, 02' and 92; The potentials are supplied over potential dropping resistors 01, 81, and 81", and appear across grid bias resistors flB, 86', and 86". The grids 90, 90, and 90'. have bias voltages due to cathode resistors 94,94, and 94" respectively; These bias voltages are modified by the voltages appearing across resistors 86, 86, and 86" respectively. The latter voltages are in turn the positive with respect to ground, but is positive or negative with respect to the cathode of tube' I00 depending upon whether grid 90 of tube 92 is positive or negative. The grids of tubes I00 and I00" are biased in a like manner. There is a triggering or looking circuit for each of the rectifier pairs described above. The tubes I00 and I02 of each locking stage have their anodes and control grids cross coupled by impedances and their grids biased and their anodes connected by resistances to direct current potential sources in :a well known manner. The anodes of the first stages comprising tubes I00, I02, etc. are coupled by condensers IM and I03 to the control grids of the second locking circuits comprising the additional tubes I04, I06, I04','I06' and I04", I06" in'a second locking circuit stage. The arrangement of the double locking circuit is suchthat 6 forthdriven by driving tubes 92,v 02 are sup-1 plied thelocking" circuits resp'ohdtothe re'- when tube I00 is conductive tube I02 isnon eon:

ductive and vice versa. When tube I00 becomes conductive its anode potential drops and this drop of'potent'i'al is applied by condenser IM to the control grid of locking tube I04 sothatthis tube becomes non conductive. The electrodes of tubes I04 and I06 are cross coupled in a conven- 'fional manner so that when tube I04 is non-conductive tube I06 is conductive and vice versa. IMoreover since when tube I00 became conductive its anode potential dropped, the anode potential of the other tube I02 of this locking circuit rises :and this rise in potential is applied by capacitor I 03 to the control grid of the tube I06 to make the same conductive. The application of the potentials through the condensers WI and I03 operates to make the triggering action very abrupt especially on the ends of the potential variations at the anodes of tubes I00 and I02 so that in the double'trigger arrangement quick action is obtained. As a matter of fact the action is so fast that asv pointed out in application Serial No. '70

632,978 the gating tubes may be turned on and of! at a rate higher than'the keying rate so that :marks may be accepted from one receiver and spaces from another receiver.

Qorresponding locking circuit arrangementsin sultan'ts'; derived in "units SSf'S'S', and .S"S by comparison of the strength oi the signals froni two of the receiver sets. I

The output from the second locking circuit in cluding tubes I04 and I06'is supplied by'resistors H3 and H5 connected between adjacent terminals of resistors I08 and I09 andadjacent termi nals of resistors H0 and III respectively and ground. The differential potential'variations at the said junction points between iresistors' I00 and I09 and H0 and III are large as compared to the variations of the driving potentials at the grid of tube 92; The voltage ,say' between re; sistors I08 and I09 is always positivewith respect to ground,'but may be positive or negative with respect to the cathode of tube I04 depending upon whether tube I04 is conducting or non conducting. This is pointed. out in detail in School: et al; application #632,978 filed December 5, 1945 and in Atwood U. S. application Serial #618,761 filed September 26, 1945, now abandoned. These potentials which vary differentially are applied by 'resistors II! and H9 etc, to the control grids of tem inthis envelope is coupled tothe junction point between resistors H0" and III"; of the locking or double trigger circuit controlled by driver tube 92". The two electron discharge devices tube" I20 are correspondingly excited one by the output appearing between resistors I08 and I00' j' the other by output appearing be tween the resistors [I0 and III. The control grids' of the electron discharge devicesin env'e lope I20" are connected one to the junction point between resistors I08' and I09f the other to the junction point between resistors fIIDT and III.

Thej-tubes I20, I20 and I20", each operate to """compare the state of stabilitygof a difierent pair of the diijferentially'operating triggering circuits}; The triggering"'circuits are 'in turn drivenby potentials derived by opposing the outputs of two of the amplifier and detector systems in the paired; rectifier circuits. The tube pair I20 is called a coincidence tube pair since the state of conductivity of each tube depends on the state of stability of the two locking circuits connectedto each, tube and a particular coin; cidence of'the state of stability of. the two locking circuits is required in order that the coinci; dence tubepair may make the gate tube controlled by it conductive. The coincidence tu e p irs have their anodes tied'together and connected by resistors to the control grids 29, 20' and 29" of the gating tubes gate A, gate B and gate C so' that the sum of the potentials at the" anodes of the tubes at I20 is applied by bias limiting resistor I28 to the control grid 20 of the tube gate A. The anodes of the tubes at I20 are tied together and connected by bias limiting resistor I28 to the control grid 20 of the tube gate B. The'anodes of the tube, at I20" are tied together and connected by bias limiting resistor I28", to the contrdlgrid 29f of thefltube ate C. The cathodes of the double 'triode tubes I20, I20 and I20" are tied together and 102" and so 7 connected to a pointoiipotentiometenPR which 9 three receivers and translate the same for use in recording apparatus not shown. No matter which signal of the three is strongest, only the strongest is passed by the gate tubes.

Since the problem is to select the strongest of three signals it might be suggested that the three diflerential voltages developed by using the six rectifier tubes could be used. However these three voltages cannot be used directly to control the gate tubes because two of them may be positive at the same time, although one of the two may be more positive than the other. By using these three differential voltages to control three trigger circuits. six voltages are obtained. Each of these voltages has two possible magnitudes (depending upon which way the triggers are flipped) each of which is relatively fixed in value. There are now six voltages which change between fixed values instead of three voltages which vary over wide limits in accordance with the signal strength. By comparing these six voltages in pairs with coincidence tube pairs, three voltages are again obtained. However, of these three voltages. only one will be of appropriate magnitude to turn on the gate tube which corresponds to the strongest signal.

If the potential at the anode end of resistor I28 becomes too positive. diode tube I 40 will be excited by'thispotential through bias limiter resistor I28 and will draw current through said resistor when its anode potential becomes more positive than its cathode potential as set by adjustment of the connections to potentiometer PR. In this manner, the bias limiter tube I 40 and limiter resistance 128 in combination limits the amount that the voltage at the grid 29 can go in the positive direction and sets it to the proper voltage for. the gate A tube bias for the conducting condition. Adiustment of the direct current bias on the control grid '29, the magnitude of the signal output from the detector in 20, etc.,' determine the minimum control potential on the grid 29 at which the tube gate A is cut ofi. Bias limiting resistors I28 and I28" and bias clamp tubes I40? and III!" are connected to the grids 29' and 29" of gate tubes B and C for similar purposes.

What is claimed is: v

1. The method of translating signals that may be subjected to fading eifects which includes these steps, deriving from signals picked up at diil'erent points a plurality of currents corresponding to said signals to be translated, said derived currents being of intensities which may change relatively to each other from time, to time. producing by means of each derived current two potentialseach having strong direct current components the magnitudes of which depend on the intensities of the derived currents by means of which they are produced, opposing each produced potential by a potential produced by means of another of said derived currents to produce resultant potentials corresponding in number to the number of derived currents and of polarities depending on the relative magnitudes of the derived currents, two of which potentials may be of like sign, deriving from each of the said resultant potentials pairs of voltages the voltages of which vary differentially each between two substantially fixed values, and effectively summing in new pairs each voltage of each of said pairs with a different one of the voltages of the remaining of said pairs to derive resultant control potentials of different magnitudes, there being a control potential for each of said derived currents.

2."I'he method of translating received signals that may be subjected to fading effects which includes these steps, deriving a plurality of currents corresponding to said signal to be translated, said derived currents being of magnitudes which may change relatively to each other from time to time, producing by means of each derived current two potentials each having strong direct current components the magnitudes of which depend on the magnitudes of the derived currents by means of which they are produced. opposing each produced potential by a potential produced by means of another of said derived currents to produce resultant potentials corresponding in number to the number of derived currents and of polarities depending on' the relative magnitudes of the derived currents, deriving from each of the said resultant potentials pairs of potentials of opposed polarity, the polarities of which pairs change differentially with changes in polarity of the said resultant potentials, effectively summing in new pairs each potential of each of said pairs with a diiferent one of the potentials of the remaining of said pairs to derive resultant control potentials of different magnitudes, there being a control potential for each of said derived currents, and controlling the translation of each of said derived currents by a different one of said control potentials.

3. In a diversity system, means for deriving a plurality of voltages varying in accordance with the same signal, said voltages being of average magnitudes which may change relatively to each other from time to time, means for selecting only the best of said voltages for recording including an electronic gating device for each of said voltages, each gating device having an input excited by one only of said voltages, a common output impedance for all of the gating devices, means for deriving from each of said voltages two potentials having strong direct current components the magnitudes of which depend on the strengths of the voltages from which they are derived, means for opposing in pairs each derived potential by a potential derived from another of said voltagesto produce resultant potentials corresponding in number to the number of said voltages and of polarities depending on the relative magnitudesof the opposed potentials, apparatus excited by said resultant potentials to produce for each thereof in pairs other potentials of opposite polarities the polarities of which reverse when the polarities of said resultant potentials change, apparatus for summing in new pairs each of said other potentials with a remaining other potential to provide a control potential for each of the summed pairs of potentials, and means for controlling the conductivities of said gating devices each by one of said control potentials and in such a manner as to bias all of said gating devices to cut off except that gating device ex cited by the best of said first mentioned voltages. 4. In a diversity system, means. for deriving a plurality of-versions of a signal to be recorded. said versions being of magnitudes which may change relatively to each other from time to time, means for selecting only the strongest signal for recording including an electronic gating device for each version of said signal, a common output impedance for all of the gating devices. means for deriving from each version of the signal two potentials having strong direct current components the magnitudes of which depend on the magnitudes of the signal versions from which they are derived, means for opposing each derived potential by a potential derived from another versionof said signal to produce resultant potentials corresponding in number to the number of signal versionsand of absolute magnitudes and polarities depending on the relative magnitudes of the opposed potentials, a locking circuit controlled by each of the resultant potentials, said locking circuits each having output impedances wherein are developed difierentially varying potentials, means for producing control potentials which are of a magnitude representing the summation in pairs of each I of the potentials developed by each locking circuit and a potential developed by-another locking circuit, there being arcontrol potential for-each version of said signal and for each gating device, and means for applying each of said control potentials to the gating device to be controlledthereby.

5. In a diversity system, at least three signal receiver, .amplifier'and detector systems, an electronic gating device for each of said receivers, each device having a control electrode coupled tothe output of the detector therefor,'said gating devices each having output electrodes, a common utilization circuit coupled to all of said output electrodes, means for comparing the strength of signal energy from each receiver with the strength of signal energy from the others of said receivers and for deriving a resultant potential, a tripping circuit for each receiver, said tripping circuits having output impeda'nces across which differentially varying potentials are produced, a driving electronic device for each tripping circuit, means for controlling each of the driving devices by a difierent one of said potentials derived by comparing the signal strengths, a pair of electronic coincidence devices for each tripping circuit, each coincidence device having a control electrode and an output electrode, a coupling between the output electrodes of each of the coincidence device pairs and -a different one of said gating devices, means for applying the potential appearing across one output impedance of each tripping circuit to the control electrode of a different one of the coincidence devices of each pair, and means for applying the potential appearing across the other output imedance of each tripping circuit to the control electrode of a different one of the remaining coincidence devices of said pairs.

6. In a diversity system, at least three signal pick up amplifier and detector systems, an electronic gating device for each of said detectors, each gating device having a control electrode coupled to the output of the detector therefor, said devices each having output electrodes in a common utilization circuit coupled to all of said output electrodes, a .diiterential detector for comparing the strength of signal energy from each pick up with the strength of signal energy from each other .ofsaid pick ups and for deriving .a resultant potential, the differential detectors-corresponding in number to the signal systems, a tripping circuit for each system, said tripping circuits having output impedances across which difierentia lly varying potentials are produced, a driving electronic device for each tripping circuit, means --for controlling each of the driving devices by-a different one of said potentials derived by comparing the signal strengths, a pair of eiectronic-coincidence devices for eachtr-ipping circuit, each coincidence device having a control electrode and an output electrode, acoupling be" tween the outputelcctrodes of eachof the coincidence device pairs and a difierent one of said gating devices, means for applying the potential appearing across one output impedance of each trippingcircuit to the control electrode of a different one of the coincidence devices of each pair, and means for applying the potential appeering across the other output impedance of each tripping circuit to the control electrode ofa different one of 'theremaining coincidence devices of said pairs.

7. In a diversity system,-at least three receivers each including signal pick up and amplifying and translating means, an electronic gating device for each of said receivers, each device having a control electrode coupled to the translating means, said devices each havingoutput electrodes connected to a-common output circuit, apparatus for generating and opposing in pairs potentials the magnitudes of which represent the strengths of the several versions of the signals at the .pick up devices, the two potentials of each pair each representing thestrength of a different version of the signals, there being two potentials generated for 'e'achlsignal version, apparatus for deriving "a resultant potential from each pair of opposed p'o-tentials, there bein a resultant for each version of said signal, said resultants being of polarities depending on the relative strengthsoi the signal versions, apparatus for each resultant actuated by the respective resultants to provide pairs of potentials of opposed polarities which change polarity differentially as the polarity of the resultant changes, apparatus for summing, in new pairs, one potential of each of said lastnamedpairsof potentials with one of the potentials of anotherof said last-named pairs, of p0 tentlals to derive a control potential for each of the summed pairs, and means for using each of 'said 'control potentials 'to control the operatlveness of an appropriate one of said gating devices.

JOHN B. ATWOOD.

REFERENCES CITED The following references are of record in the file of this patent:

UNITED STATES PATENTS Number 'Name 'Date 2,004,126 Moore -f June 11,1935 "2,299,425 Hansell July '15, 1941 2,290,992 Peterson July 28, I942 2,414,111 Lyons Jan. 14, 1947 

